Good point, I withdraw my suggestion. I think it's unfortunate for Python to have traps like this, but I don't see a nice way to protect users while still letting people do whatever they want. On Thu, Apr 21, 2016 at 3:03 AM Neil Girdhar <mistersheik@gmail.com> wrote:

I guess that's fair.

On Thu, Apr 21, 2016 at 2:36 AM Nick Coghlan <ncoghlan@gmail.com> wrote:

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On 21 April 2016 at 12:51, Neil Girdhar <mistersheik@gmail.com> wrote:

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Sometimes users inherit from builtin types only to find that their overridden methods are not called. Instead of this being a trap for unsuspecting users, I suggest overriding the __new__ method of these types so that it will raise with an informative exception explaining that, e.g., instead of inheriting from dict, you should inherit from UserDict.

I suggest this modification to any Python implementation that has special versions of classes that cannot easily be extended, such as CPython. If another Python implementation allows dict (e.g.) to be extended easily, then it doesn't have to raise.

Builtins can be extended, you just have to override all the methods where you want to change the return type:

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from collections import defaultdict, Counter, OrderedDict issubclass(defaultdict, dict) True issubclass(Counter, dict) True issubclass(OrderedDict, dict) True

This isn't hard as such, it's just tedious, so it's often simpler to use the more subclass friendly variants that dynamically look up the type to return and hence let you get away with overriding a smaller subset of the methods.

Cheers, Nick.

-- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia